Graphene refers to a two-dimensional thin layer with honeycomb structure, comprising one or more layers of carbon atoms. When the carbon atoms are chemically bonded due to an sp2 hybrid orbital, a two-dimensional plane of a hexagonal carbon mesh is provided. Carbon has four outermost electrons, and the four electrons are hybridized to participate in bonding. A method of bonding carbon atoms includes sp3 bonding and sp2 bonding. Carbon atoms bonded using only sp3 bonding correspond to a diamond having a square shape, and carbon atoms bonded using only sp2 bonding correspond to graphite or graphene, a layer of graphite. For example, electrons which should be present in an s orbital and a p orbital have an sp2 or sp3 hybrid orbital corresponding to a combination of the s and p orbitals. The sp2 hybrid orbital has one electron in the s orbital and two electrons in the p orbital, and thus has a total of three electrons. In this case, the electrons have equal energy levels. Having the hybrid orbital is more stable compared to separately having the s and p orbitals. An aggregate of carbon atoms having a planar structure due to sp2 bonding is graphene, and the thickness of a monolayer thereof corresponds to the size of one carbon atom, e.g., about 0.3 nm. Graphene has properties of metal. For example, graphene has conductivity in a layer direction, has excellent thermal conductivity, and has a high mobility of charge carriers, and thus may be used to implement a high-speed electronic device. It is known that electron mobility of a graphene sheet has a value of about 20,000 to 50,000 cm2/Vs.
Since a research group of the University of Manchester, UK has published a method of producing a thin carbon layer having a thickness of one atom, from graphite in 2005, graphene has become one of the most attractive study items in physics because research can be conducted on the unique quantum-Hall effect of graphene and conventionally impossible particle physics experiments can be indirectly implemented using graphene based on a fact that electrons have no effective mass in graphene and thus behave as relativistic particles moving at 1,000 km per sec. ( 1/300 of the speed of light).
Using a conventional silicon-based semiconductor process technology, a highly-integrated semiconductor device having a thickness equal to or less than 30 nm may not be easily manufactured because atoms of metal, e.g., gold (Au) or aluminum (Al), deposited on a substrate are thermodynamically unstable, stuck to each other, and thus incapable of obtaining a uniform thin layer if the thickness of the layer thereof is equal to or less than 30 nm, and because the density of impurities doped on silicon is not uniform at the above-mentioned nano-size. However, graphene has a possibility of solving the restriction in integration of the silicon-based semiconductor device technology. Graphene has properties of metal, has a thickness equal to or less than several nm corresponding to an electron shielding thickness, and thus has an electric resistance which varies due to a charge density changed depending on a gate voltage. Using the above characteristics, a metal transistor may be implemented. Furthermore, due to a high mobility of charge carriers, a high-speed electronic device may be implemented. In addition, charges of the charge carriers may be changed from electrons to holes depending on the polarity of the gate voltage, and application to a variety of fields may be expected.
Currently, a method of obtaining graphene is classified into the following three.
The first method is a micro cleavage method using cellophane tape. This method has been developed by a research team of the University of Manchester, UK, and researchers use this method due to simplicity thereof. In this method, the thickness of graphite may be reduced by repeatedly peeling graphite using cellophane tape, and a thin graphite layer obtained as described above is transferred onto a substrate. Alternatively, a thin graphite layer is obtained by rubbing graphite on a substrate like rubbing chalk on a blackboard. However, this method depends on the quality of the adhesive tape, and electrodes may not be easily patterned using electron-beam lithography due to a large number of useless and thick graphite particles.
The second method is a method of epitaxially growing graphene by pyrolyzing silicon carbide (SiC) under high vacuum. This epitaxial growth method is a method of producing graphene using carbon atoms remaining on the surface of SiC after silicon is sublimated from the surface at high vacuum and high temperature, e.g., in a molecular beam epitaxy (MBE) system. In this technology, SiC should be used as a substrate. However, this substrate does not have good performance to be used as an electronic material.
The third method is a method using chemical peeling of a graphite compound. However, in this method, only a piece of graphite having a thickness of several hundred nanometers may be obtained and graphene may not be obtained. In addition, a chemical material inserted between graphite layers may not be completely removed and thus a large number of defects may be caused.
The fourth method is a method of growing graphene on a metal substrate using chemical vapor deposition (CVD). However, in this method, graphene may not be oriented in a direction due to growth characteristics thereof. The largest obstacle to commercialization of graphene electronic devices is not a fact that a bandgap may not be easily provided due to material characteristics and thus a logic circuit may not be easily configured, but is a fact that a large-area single crystal, e.g., silicon, may not be easily found. It is currently known that initial uniformity of a graphene layer is controllable by growing graphene on almost melted liquid-state copper. However, even in this case, directivities are not equal and thus growing of a large-area graphene single crystal is impossible. A new idea for producing monocrystalline graphene is required to newly develop electronic devices using graphene.